The present invention relates to an input stage for digital signals, as used as a peripheral input interface for process computers and programmable controllers. Such peripheral modules are needed in one installation in a relatively large number and one attempts to keep the power loss, which develops in them and is converted into heat, as low as possible, so that a non-ventilated operation with natural air cooling is possible without having to supply cooling air.
The German Patent No. 28 14 856 discloses a trigger circuit for a light-emitting diode, which circuit is linked via a current limiting resistor to two input terminals capable of receiving signal voltage. In this case, the light-emitting diode is triggered by a Schmitt trigger, which is likewise connected up to the input voltage. The Schmitt trigger short-circuits the light-emitting diode on the output side in dependence upon the value of the signal voltage. Instead of a current limiting resistor, this patent proposes using an arrangement which receives a constant current. This provides the advantage of minimal power loss when voltage values vary. This known trigger circuit can be used for the isolated transmission of digital signals with an optocoupler, for example, as an input device for digital control systems.
To achieve economical manufacturing with a large number of pieces it is advantageous to implement input stages using of modern semiconductor technology, that is to realize the input stages as integrated circuits. The power loss arising in the integrated circuit under field conditions must be kept low enough, so that this circuit is not destroyed by the generation of heat. In particular, when several similar input ports are to be combined in a shared circuit, this power-loss limit is very quickly exceeded (for example, in the case of input stages intended for programmable controllers) and can no longer be observed without extensive measures in many applications. As a result of the minimum current stipulated for compatibility reasons by international standards for the "on" state of the input stage, expensive power housings for the circuit are necessary, especially in the case of a multi-channel configuration.